LibWeb/CSS: Add valid default font if no font can be found

- Before: If a font cannot computed based on style values, e.g. if the
          family is incorrect (try 'lolol'), then the list of found
          fonts contains only the default emoji font.
- After:  The default font of the Font plugin is added to the top of
          an empty list of computed fonts. This way, we're failing
          gracefully on author error, and we still display emojis :]m

Bonus: renamed `found_font` (misleading name; seems artifact) to
       `last_resort_font` (what it is)

Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -2090,12 +2090,18 @@ RefPtr<Gfx::FontCascadeList const> StyleComputer::compute_font_for_style_values(
             font_list->extend(*other_font_list);
     }
 
+    auto fallback_font = Platform::FontPlugin::the().default_font().with_size(font_size_in_pt);
+    if (font_list->is_empty()) {
+        // FIXME: Opinion: Platform::FontPlugin::the().default_font() (now: 'sans') should match the 'html' (top) style in Default.css ('serif')
+        font_list->add(*fallback_font);
+    }
+
     if (auto emoji_font = Platform::FontPlugin::the().default_emoji_font(font_size_in_pt); emoji_font) {
         font_list->add(*emoji_font);
     }
 
-    auto found_font = StyleProperties::font_fallback(monospace, bold);
-    font_list->set_last_resort_font(found_font->with_size(font_size_in_pt));
+    auto last_resort_font = StyleProperties::font_fallback(monospace, bold);
+    font_list->set_last_resort_font(last_resort_font->with_size(font_size_in_pt));
 
     return font_list;
 }